The present invention relates to the manufacture of semiconductor devices. More specifically, the invention relates to lithographic exposure in the manufacture of semiconductor devices.
Wafer steppers are called the working horses of optical lithography for semiconductor manufacturing. Recently, step and scan systems (scanners) were introduced into manufacturing, which do not expose an image by one single step (as is done by steppers), but instead use a synchronized scanning of the wafer and the reticle through a fixed slit within the associated optics. Step and scan systems are especially used for printing patterns of 200 nm (nanometers) or less in advanced semiconductor manufacturing.
Critical dimensions (CD) of a semiconductor device are the widths of lines and spaces of critical circuit patterns as well as areas of contacts of the semiconductor device. To obtain a good operating semiconductor device, the critical dimensions of the semiconductor device must be very accurate. The CD distribution for gates of transistors must be especially tight over the complete exposure field of the scanners. Every nanometer of a tighter CD-distribution gives a higher operation speed for the resulting semiconductor device.
In practice, there are many contributing factors that cause a greater deviation from the target CD, and therefore a less tight CD-distribution. Some of these factors are the variations in resist thickness and development uncertainties (like variations in development time or temperature). Imperfections in the lens and reticle and also synchronization errors between the wafer and the reticle carriers (stages) add to the deviation from the target CD.
One method of reducing CD-distribution known in the art is optical proximity correction.
The transition from using steppers to scanners also has helped to reduce the effect of imperfections in the reticle on variations from the target CD. In addition, lenses and reticles have been improved to reduce CD-variations. However, reticles still have imperfections (e.g. typical CD-variations of 30 nm on the reticle), so that a 4 times reduction ratio of the lens would cause a CD variation of about 7.5 nm on the wafer. In addition, reticle errors can get enhanced by nonlinearities of the lithographic process. This effect is described by the Mask Error Enhancement Factor (MEEF). In an example, a MEEF of 2 would lead to a reticle induced CD variation of 15 nm instead of 7.5 nm. Defocus further may increase the MEEF. In addition, it is still impossible to manufacture error free lenses and exposure systems. As a result, every reticle and lens combination has a distinctive error signature, which increases CD-distribution. The error that is created by the reticle and lens combination is repeated from die to die and wafer to wafer.
It would be advantageous to be able to compensate for exposure system error, such as reticle error and lens error, to reduce CD-distribution.
It is an object of the invention to compensate for exposure system error to reduce CD-distribution.
Accordingly, the foregoing object is accomplished by creating from the resist exposure system an exposure pattern with a distribution of the exposure pattern across an exposure field, measuring the distribution of the exposure pattern across the exposure field, and correcting exposure from the resist exposure system according to the measured distribution of the exposure pattern across the exposure field.
Other features of the present invention are disclosed or apparent in the section entitled: xe2x80x9cDETAILED DESCRIPTION OF THE INVENTION.xe2x80x9d